3GPP LTE employs Orthogonal Frequency Division Multiple Access (OFDMA) as a downlink communication scheme. In radio communication systems to which 3GPP LTE is applied, base stations transmit synchronization signals (i.e., Synchronization Channel: SCH) and broadcast signals (i.e., Broadcast Channel: BCH) using predetermined communication resources. Meanwhile, each terminal finds an SCH first and thereby ensures synchronization with a base station. Subsequently, the terminal reads BCH information to acquire base station-specific parameters (see, Non-Patent Literatures (hereinafter, abbreviated as NPL) 1, 2 and 3).
In addition, upon completion of the acquisition of the base station-specific parameters, each terminal sends a connection request to the base station to thereby establish a communication link with the base station. The base station transmits control information via Physical Downlink Control Channel (PDCCH) as appropriate to the terminal with which a communication link has been established.
The terminal performs “blind-determination” on each of a plurality of pieces of control information included in the received PDCCH signals (i.e., Downlink (DL) Assignment Control Information: also referred to as Downlink Control Information (DCI)). To put it more specifically, each piece of the control information includes a Cyclic Redundancy Check (CRC) part and the base station masks this CRC part using the terminal ID of the transmission target terminal. Accordingly, until the terminal demasks the CRC part of the received piece of control information with its own terminal ID, the terminal cannot determine whether or not the piece of control information is intended for the terminal. In this blind-determination, if the result of demasking the CRC part indicates that the CRC operation is OK, the piece of control information is determined as being intended for the terminal.
Moreover, in 3GPP LTE, Automatic Repeat Request (ARQ) is applied to downlink data to terminals from a base station. To put it more specifically, each terminal feeds back response signals indicating the result of error detection on the downlink data to the base station. Each terminal performs a CRC on the downlink data and feeds back Acknowledgment (ACK) when CRC=OK (no error) or Negative Acknowledgment (NACK) when CRC=Not OK (error) to the base station as response signals. An uplink control channel such as Physical Uplink Control Channel (PUCCH) is used to feed back the response signals (i.e., ACK/NACK signals (hereinafter, may be referred to as “A/N,” simply)).
The control information to be transmitted from a base station herein includes resource assignment information including information on resources assigned to the terminal by the base station. As described above, PDCCH is used to transmit this control information. The PDCCH includes one or more L1/L2 control channels (L1/L2 CCH). Each L1/L2 CCH consists of one or more Control Channel Elements (CCE). To put it more specifically, a CCE is the basic unit used to map the control information to PDCCH. Moreover, when a single L1/L2 CCH consists of a plurality of CCEs (2, 4 or 8), a plurality of contiguous CCEs starting from a CCE having an even index are assigned to the L1/L2 CCH. The base station assigns the L1/L2 CCH to the resource assignment target terminal in accordance with the number of CCEs required for indicating the control information to the resource assignment target terminal. The base station maps the control information to physical resources corresponding to the CCEs of the L1/L2 CCH and transmits the mapped control information.
In addition, CCEs are associated with component resources of PUCCH (hereinafter, may be referred to as “PUCCH resource”) in a one-to-one correspondence. Accordingly, a terminal that has received an L1/L2 CCH identifies the component resources of PUCCH that correspond to the CCEs forming the L1/L2 CCH and transmits response signals to the base station using the identified resources. However, when the L1/L2 CCH occupies a plurality of contiguous CCEs, the terminal transmits the response signals to the base station using a PUCCH component resource corresponding to a CCE having a smallest index among the plurality of PUCCH component resources respectively corresponding to the plurality of CCEs (i.e., PUCCH component resource associated with a CCE having an even numbered CCE index). In this manner, the downlink communication resources are efficiently used.
Moreover, 3GPP LTE employs a scheduling scheme of assigning radio resources in a constant cycle for packet data in VoIP, streaming, and the like involving a transmission rate that is constant to some extent, instead of employing a best-effort scheduling scheme (dynamic scheduling), which dynamically assigns radio resources to achieve higher efficiency. This scheduling scheme is referred to, for example, persistent scheduling or semi-persistent scheduling (SPS). In SPS, activation and release are indicated through a PDCCH. Once SPS is activated, a base station transmits a Physical Downlink Shared Channel (PDSCH) in a constant cycle and no longer indicates a PDCCH with respect to the PDSCH scheduled by SPS. In SPS, since the base station and a terminal perform transmission and reception at known transmission timing as described above, downlink scheduling information (DL scheduling information) can be reduced, which in turn makes it possible to effectively utilize downlink radio resources. During SPS transmission, the terminal feeds back response signals to the base station. This feedback of the response signals is performed using a PUCCH resource corresponding to one of four PUCCH resource indexes (nPUCCH(1)) that are set in advance in a one-to-one correspondence with (two-bit) values of a transmission power control (TPC) command in the PDCCH indicating the activation of SPS.
As illustrated in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread using a Zero Auto-correlation (ZAC) sequence having the characteristic of zero autocorrelation in time-domain, a Walsh sequence and a discrete Fourier transform (DFT) sequence, and are code-multiplexed in a PUCCH. In FIG. 1, (W1, W1, W2, W3) represent a length-4 Walsh sequence and (F0, F1, F2) represent a length-3 DFT sequence. As illustrated in FIG. 1, ACK or NACK response signals are primary-spread over frequency components corresponding to 1 SC-FDMA symbol by a ZAC sequence (length-12) in frequency-domain. To put it more specifically, the length-12 ZAC sequence is multiplied by a response signal component represented by a complex number. Subsequently, the ZAC sequence serving as the response signals and reference signals after the primary-spread is secondary-spread in association with each of a Walsh sequence (length-4: W0-W3 (may be referred to as Walsh Code Sequence)) and a DFT sequence (length-3: F0-F2). To put it more specifically, each component of the signals of length-12 (i.e., response signals after primary-spread or ZAC sequence serving as reference signals (i.e., Reference Signal Sequence) is multiplied by each component of an orthogonal code sequence (i.e., orthogonal sequence: Walsh sequence or DFT sequence). Moreover, the secondary-spread signals are transformed into signals of length-12 in the time-domain by inverse fast Fourier transform (IFFT). A CP is added to each signal obtained by IFFT processing, and the signals of one slot consisting of seven SC-FDMA symbols are thus formed.
The response signals from different terminals are spread using ZAC sequences each corresponding to a different cyclic shift value (i.e., index) or orthogonal code sequences each corresponding to a different sequence number (i.e., orthogonal cover index (OC index)). An orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. In addition, an orthogonal code sequence is referred to as a block-wise spreading code in some cases. Thus, base stations can demultiplex the code-multiplexed plurality of response signals using the related art despreading and correlation processing (see, NPL 4).
However, it is not necessarily true that each terminal succeeds in receiving downlink assignment control signals because the terminal performs blind-determination in each subframe to find downlink assignment control signals intended for the terminal. When the terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal would not even know whether or not there is downlink data intended for the terminal on the downlink component carrier. Accordingly, when a terminal fails to receive the downlink assignment control signals intended for the terminal on a certain downlink component carrier, the terminal generates no response signals for the downlink data on the downlink component carrier. This error case is defined as discontinuous transmission of ACK/NACK signals (DTX of response signals) in the sense that the terminal transmits no response signals.
In 3GPP LTE systems (may be referred to as “LTE system,” hereinafter), base stations assign resources to uplink data and downlink data, independently. For this reason, in the 3GPP LTE system, terminals (i.e., terminals compliant with LTE system (hereinafter, referred to as “LTE terminal”)) encounter a situation where the terminals need to transmit uplink data and response signals for downlink data simultaneously in the uplink. In this situation, the response signals and uplink data from the terminals are transmitted using time-division multiplexing (TDM). As described above, the single carrier properties of transmission waveforms of the terminals are maintained by the simultaneous transmission of response signals and uplink data using TDM.
In addition, as illustrated in FIG. 2, the response signals (i.e., “A/N”) transmitted from each terminal partially occupy the resources assigned to uplink data (i.e., Physical Uplink Shared CHannel (PUSCH) resources) (i.e., response signals occupy some SC-FDMA symbols adjacent to SC-FDMA symbols to which reference signals (RS) are mapped) and are thereby transmitted to a base station in time-division multiplexing (TDM). In FIG. 2, however, “subcarriers” in the vertical axis of the drawing are also termed as “virtual subcarriers” or “time contiguous signals,” and “time contiguous signals” that are collectively inputted to a discrete Fourier transform (DFT) circuit in a SC-FDMA transmitter are represented as “subcarriers” for convenience. To put it more specifically, optional data of the uplink data is punctured due to the response signals in the PUSCH resources. Accordingly, the quality of uplink data (e.g., coding gain) is significantly reduced due to the punctured bits of the coded uplink data. For this reason, base stations instruct the terminals to use a very low coding rate and/or to use very large transmission power so as to compensate for the reduced quality of the uplink data due to the puncturing.
Meanwhile, the standardization of 3GPP LTE-Advanced for realizing faster communications than 3GPP LTE has started. 3GPP LTE-Advanced systems (may be referred to as “LTE-A system,” hereinafter) follow 3GPP LTE systems (may be referred to as “LTE system,” hereinafter). 3GPP LTE-Advanced is expected to introduce base stations and terminals capable of communicating with each other using a wideband frequency of 40 MHz or greater to realize a downlink transmission rate up to 1 Gbps or above.
In the LTE-A system, in order to simultaneously achieve backward compatibility with the LTE system and ultra-high-speed communications several times faster than transmission rates in the LTE system, the LTE-A system band is divided into “component carriers” of 20 MHz or below, which is the bandwidth supported by the LTE system. In other words, the “component carrier” is defined herein as a band having a maximum width of 20 MHz and as the basic unit of communication band. Moreover, “component carrier” in downlink (hereinafter, referred to as “downlink component carrier”) is defined as a band obtained by dividing a band according to downlink frequency bandwidth information in a BCH broadcasted from a base station or as a band defined by a distribution width when a downlink control channel (PDCCH) is distributed in the frequency domain. In addition, “component carrier” in uplink (hereinafter, referred to as “uplink component carrier”) may be defined as a band obtained by dividing a band according to uplink frequency band information in a BCH broadcasted from a base station or as the basic unit of a communication band of 20 MHz or below including a Physical Uplink Shared Channel (PUSCH) in the vicinity of the center of the bandwidth and PUCCHs for LTE on both ends of the band. In addition, the term “component carrier” may be also referred to as “cell” in English in 3GPP LTE-Advanced and may be abbreviated as CC(s).
The LTE-A system supports communications using a band obtained by aggregating several component carriers, so called “carrier aggregation.” In general, throughput requirements for uplink are different from throughput requirements for downlink. For this reason, so called “asymmetric carrier aggregation” has been also discussed in the LTE-A system. In asymmetric carrier aggregation, the number of component carriers configured for any terminal compliant with the LTE-A system (hereinafter, referred to as “LTE-A terminal”) differs between uplink and downlink. In addition, the LTE-A system supports a configuration in which the numbers of component carriers are asymmetric between uplink and downlink, and the component carriers have different frequency bandwidths.
FIG. 3 is a diagram provided for describing asymmetric carrier aggregation and a control sequence applied to individual terminals. FIG. 3 illustrates a case where the bandwidths and numbers of component carriers are symmetric between the uplink and downlink of base stations.
In FIG. 3, a configuration in which carrier aggregation is performed using two downlink component carriers and one uplink component carrier on the left is set for terminal 1, while a configuration in which the two downlink component carriers identical with those used by terminal 1 are used but uplink component carrier on the right is used for uplink communications is set for terminal 2.
Referring to terminal 1, an LTE-A base station and an LTE-A terminal included in the LTE-A system transmit and receive signals to and from each other in accordance with the sequence diagram illustrated in FIG. 3A. As illustrated in FIG. 3A, (1) terminal 1 is synchronized with the downlink component carrier on the left when starting communications with the base station and reads information on the uplink component carrier paired with the downlink component carrier on the left from a broadcast signal called system information block type 2 (SIB2). (2) Using this uplink component carrier, terminal 1 starts communications with the base station by transmitting, for example, a connection request to the base station. (3) Upon determining that a plurality of downlink component carriers need to be assigned to the terminal, the base station instructs the terminal to add a downlink component carrier. However, in this case, the number of uplink component carriers is not increased, and terminal 1, which is an individual terminal, starts asymmetric carrier aggregation.
In addition, in the LTE-A system to which carrier aggregation is applied, a terminal may receive a plurality of pieces of downlink data on a plurality of downlink component carriers at a time. In LTE-A, studies have been carried out on channel selection (also referred to as “multiplexing”), bundling and a discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) format as a method of transmitting a plurality of response signals for the plurality of pieces of downlink data. In channel selection, not only symbol points used for response signals, but also the resources to which the response signals are mapped are varied in accordance with the pattern for results of the error detection on the plurality of pieces of downlink data. Compared with channel selection, in bundling, ACK or NACK signals generated according to the results of error detection on the plurality of pieces of downlink data are bundled (i.e., bundled by calculating a logical AND of the results of error detection on the plurality of pieces of downlink data, provided that ACK=1 and NACK=0), and response signals are transmitted using one predetermine resource. In transmission using the DFT-S-OFDM format, a terminal jointly encodes (i.e., joint coding) the response signals for the plurality of pieces of downlink data and transmits the coded data using the format (see, NPL 5).
More specifically, channel selection is a technique that varies not only the phase points (i.e., constellation points) for the response signals but also the resources used for transmission of the response signals (may be referred to as “PUCCH resource,” hereinafter) on the basis of whether the results of error detection on the plurality of pieces of downlink data received on the plurality of downlink component carriers are each an ACK or NACK as illustrated in FIG. 4. Meanwhile, bundling is a technique that bundles ACK/NACK signals for the plurality of pieces of downlink data into a single set of signals and thereby transmits the bundled signals using one predetermined resource (see, NPLs 6 and 7).
The following two methods are considered as a possible method of transmitting response signals in uplink when a terminal receives downlink assignment control information via a PDCCH and receives downlink data.
One of the methods is to transmit response signals using a PUCCH resource associated in a one-to-one correspondence with a control channel element (CCE) occupied by the PDCCH (i.e., implicit signaling) (hereinafter, method 1). More specifically, when DCI intended for a terminal served by a base station is allocated in a PDCCH region, each PDCCH occupies a resource consisting of one or a plurality of contiguous CCEs. In addition, as the number of CCEs occupied by a PDCCH (i.e., the number of aggregated CCEs: CCE aggregation level), one of aggregation levels 1, 2, 4 and 8 is selected according to the number of information bits of the assignment control information or a propagation path condition of the terminal, for example. This resource is associated in a one-to-one correspondence with and implicitly assigned to a CCE index, and thus may be referred to as implicit resource.
The other method is to previously indicate a PUCCH resource to each terminal from a base station (i.e., explicit signaling) (hereinafter, method 2). To put it differently, each terminal transmits response signals using the PUCCH resource previously indicated by the base station in method 2. This resource is explicitly indicated in advance by the base station, and thus may be referred to as explicit resource.
In addition, as illustrated in FIG. 4, one of the two downlink component carriers is paired with one uplink component carrier to be used for transmission of response signals. The downlink component carrier paired with the uplink component carrier to be used for transmission of response signals is called a primary component carrier (PCC) or a primary cell (PCell). In addition, the downlink component carrier other than the primary component carrier is called a secondary component carrier (SCC) or a secondary cell (SCell). For example, PCC (or PCell) is the downlink component carrier used to transmit broadcast information about the uplink component carrier on which response signals to be transmitted (e.g., system information block type 2 (SIB 2)).
Meanwhile, in channel selection, a PUCCH resource in an uplink component carrier associated in a one-to-one correspondence with the top CCE index of the CCEs occupied by the PDCCH indicating the PDSCH in PCC (PCell) (i.e., PUCCH resource in PUCCH region 1 in FIG. 4) is assigned (implicit signaling).
Next, a description will be provided regarding ARQ control using channel selection when the asymmetric carrier aggregation described above is applied to terminals with reference to FIGS. 4, 5 and 6.
In a case where a component carrier group (may be referred to as “component carrier set” in English) consisting of downlink component carrier 1 (PCell), downlink component carrier 2 (SCell) and uplink component carrier 1 is configured for terminal 1 as illustrated in FIG. 4, after downlink resource assignment information is transmitted via a PDCCH of each of downlink component carriers 1 and 2, downlink data is transmitted using the resource corresponding to the downlink resource assignment information.
In channel selection, when terminal 1 succeeds in receiving the downlink data on component carrier 1 (PCell) but fails to receive the downlink data on component carrier 2 (SCell) (i.e., when the result of error detection on component carrier 1 (PCell) is an ACK and the result of error detection on component carrier 2 (SCell) is a NACK), the response signals are mapped to a PUCCH resource in PUCCH region 1 to be implicitly signaled, while a first phase point (e.g., phase point (1, 0) and/or the like) is used as the phase point of the response signals. In addition, when terminal 1 succeeds in receiving the downlink data on component carrier 1 (PCell) and also succeeds in receiving the downlink data on component carrier 2 (SCell), the response signals are mapped to a PUCCH resource in PUCCH region 2 while the first phase point is used. That is, in the configuration including two downlink component carriers with a transmission mode that supports only one transport block (TB) per downlink component carrier, the results of error detection are represented in four patterns (i.e., ACK/ACK, ACK/NACK, NACK/ACK, and NACK/NACK). Hence, the four patterns can be represented by combinations of two PUCCH resources and two kinds of phase points (e.g., binary phase shift keying (BPSK) mapping).
In addition, when terminal 1 fails to receive DCI on component carrier 1 (PCell) but succeeds in receiving downlink data on component carrier 2 (SCell) (i.e., the result of error detection on component carrier 1 (PCell) is a DTX and the result of error detection on component carrier 2 (SCell) is an ACK), the CCEs occupied by the PDCCH intended for terminal 1 cannot be identified. Thus, the PUCCH resource included in PUCCH region 1 and associated in a one-to-one correspondence with the top CCE index of the CCEs cannot be identified either. Accordingly, in this case, in order to report an ACK, which is the result of error detection on component carrier 2, the response signals need to be mapped to an explicitly signaled PUCCH resource included in PUCCH region 2 (may be referred to as “to support implicit signaling,” hereinafter).
To be more specific, FIG. 5 and FIG. 6 each illustrate mapping of patterns for the results of error detection in the configuration including two downlink component carriers (one PCell and one SCell) with:
(a) the transmission mode that supports only 1 TB for each downlink component carrier;
(b) the transmission mode that supports only 1 TB for the downlink component carrier of PCell and the transmission mode that supports up to 2 TBs for the downlink component carrier of SCell;
(c) the transmission mode that supports up to 2 TBs for the downlink component carrier of PCell and the transmission mode that supports only 1 TB for the downlink component carrier of SCell; and
(d) the transmission mode that supports up to 2 TBs for each downlink component carrier. FIG. 7 illustrates the mapping of each of FIG. 5 and FIG. 6 in the form of a table (hereinafter, may be referred to as “mapping table” or “transmission rule table”).
For downlink data channel (Physical Downlink Shared Channel: PDSCH) transmission in PCell, the PUCCH resource indicating method disclosed in NPL 8 uses an implicit resource when dynamic scheduling is used for PCell. Meanwhile, when SPS is used for PCell, this method uses one of four PUCCH resources set in advance in a one-to-one correspondence with values of a TPC command for PUCCH that is included in the PDCCH indicating the activation of SPS, similarly to 3GPP LTE. For PDSCH transmission in SCell, this method uses an implicit resource when a PDCCH corresponding to a PDSCH in SCell is placed in PCell (hereinafter, may be referred to as “cross-carrier scheduling from PCell to SCell”) and uses an explicit resource when no cross-carrier scheduling from PCell to SCell is configured.
In the method disclosed in NPL 8, for PDSCH transmission in SCell when no cross-carrier scheduling from PCell to SCell is configured, a PDCCH corresponding to a PDSCH in SCell is placed in SCell. In such a case, if an implicit resource, which is implicitly indicated on the basis of the CCE index, is used, the CCE index of a PDCCH placed in PCell that is intended for the target terminal or a different terminal may be the same as the CCE of the PDCCH placed in SCell that is intended for the target terminal. In this case, the same PUCCH resource is indicated to both PCell and SCell, and a collision of response signals occurs unfavorably. For this reason, an explicit resource is used for PDSCH transmission in SCell when no cross-carrier scheduling from PCell to SCell is configured. On the other hand, for PDSCH transmission in SCell when cross-carrier scheduling from PCell to SCell is configured, the PDCCH corresponding to the PDSCH in SCell is placed in PCell. In this case, there is no such case where a CCE occupied by a different PDCCH intended for the same terminal or by a PDCCH intended for another terminal is used for the CCE occupied by the abovementioned PDCCH. Hence, an implicit resource can be used for the PDSCH transmission in SCell when cross-carrier scheduling from PCell to SCell is configured.
The PUCCH resource indicating method disclosed in NPL 9 uses one implicit resource for non-MIMO DCI and two implicit resources for MIMO DCI for PDSCH transmission in PCell. This method uses an explicit resource for PDSCH transmission in SCell.
In the case of NPL 9, 1 CCE includes 36 resource elements (REs), and 72 bits can be thus transmitted per CCE when QPSK mapping for each resource element is used. Non-MIMO DCI has a smaller number of bits than MIMO DCI, and thus can be transmitted using 1 CCE. In contrast, MIMO DCI has a larger number of bits than non-MIMO DCI, and is generally transmitted using 2 or more CCEs in order to reduce the error rate of PDCCH. Accordingly, in the case of NPL 9, one implicit resource is used for non-MIMO DCI in consideration of PDCCH transmission using 1 or more CCEs, whereas two implicit resources are used for MIMO DCI in consideration of PDCCH transmission using 2 or more CCEs.
If two implicit resources are used for PDCCH transmission using 1 CCE, because the implicit resources are associated in a one-to-one correspondence with the CCE indexes, 2 CCEs need to be occupied for the PUCCH resource indication although the PDCCH transmission occupies only 1 CCE. In such a case where a larger number of CCEs than the number of CCEs occupied by PDCCH are occupied for the PUCCH resource indication, a PDCCH for another terminal cannot be assigned to these CCEs, which results in restrictions on PDCCH scheduling in the base station.